The IP (Internet Protocol) Multimedia Subsystem (IMS) is a standardized architecture for providing multimedia services and voice-over-IP calls to both mobile and fixed user agents (UAs). The Session Initiation Protocol (SIP) been standardized and governed primarily by the Internet Engineering Task Force (IETF) as a protocol for setting up and managing IMS-based calls. As used herein, the terms “user agent” and “UA” might in some cases refer to mobile devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices that have telecommunications capabilities. Such a UA might consist of a UA and its associated removable memory module, such as but not limited to a Universal Integrated Circuit Card (UICC) that includes a Subscriber Identity Module (SIM) application, a Universal Subscriber Identity Module (USIM) application, or a Removable User Identity Module (R-UIM) application. Alternatively, such a UA might consist of the device itself without such a module. In other cases, the term “UA” might refer to devices that have similar capabilities but that are not transportable, such as fixed line telephones, desktop computers, set-top boxes, or network nodes. When a UA is a network node, the network node could act on behalf of another function such as a UA or a fixed line device and simulate or emulate the UA or fixed line device. For example, for some UAs, the IMS SIP client that would typically reside on the device actually resides in the network and relays SIP message information to the device using optimized protocols. In other words, some functions that were traditionally carried out by a UA can be distributed in the form of a remote UA, where the remote UA represents the UA in the network. The term “UA” can also refer to any hardware or software component that can terminate a communication session that could include, but is not limited to, a SIP session. Also, the terms “user agent”, “UA”, “user equipment”, “UE”, “UA”, and “node” might be used synonymously herein.
Some UAs communicate in a circuit switched mode, wherein a dedicated communication path exists between two endpoints. For the duration of a call or session, all data exchanged between the two endpoints travels along the single path. Some UAs have the capability to communicate in a packet switched mode, wherein a data stream representing a portion of a call or session is divided into packets that are given unique identifiers. The packets might then be transmitted from a source to a destination along different paths and might arrive at the destination at different times. Upon reaching the destination, the packets are reassembled into their original sequence based on the identifiers.
Communications that take place via circuit switching can be said to occur in the circuit switched domain and communications that take place via packet switching can be said to occur in the packet switched domain. Some UAs may be capable of communicating in both the circuit switched domain and the packet switched domain. Within each domain, several different types of networks, protocols, or technologies can be used. In some cases, the same network, protocol, or technology can be used in both domains. The wireless communication networks may be based on Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Orthogonal Frequency Division Multiplexing (OFDM), or some other multiple access scheme. A CDMA-based network may implement one or more standards such as 3GPP2 IS-2000 (commonly referred to as CDMA 1x), 3GPP2 IS-856 (commonly referred to as CDMA 1xEV-DO), or 3GPP UMTS (Universal Mobile Telecommunications System). A TDMA-based network may implement one or more standards such as 3GPP Global System for Mobile Communications (GSM) or 3GPP General Packet Radio Service (GPRS).
GSM is an example of a wireless network standard that uses only the circuit switching mode. Examples of wireless network standards that use only packet switching include GPRS, CDMA 1x EV-DO, Worldwide Interoperability for Microwave Access (WiMAX), and Wireless Local Area Network (WLAN), which might comply with Institute of Electrical and Electronics Engineers (IEEE) standards such as 802.16, 802.16e, 802.11a, 802.11b, 802.11g, 802.11n, and similar standards. Examples of wireless network standards that may use both circuit switching and packet switching modes include CDMA 1x and UMTS. SIP is an example of an application-layer protocol that can be used in a packet switched wireless network.